Postponing the execution just makes no sense. There is obviously some prerequisite that hasn't been met with the former hero; otherwise the demons would have just let him get executed and then revived him. The only rational decision is to kill him immediately without any ceremony at all, then just to be sure, incinerate the body and throw the ashes in the sewer.
I think we saw the real reason. The necromancer even said something important.
That a lower rank demon wouldn't be resurrected, but that a high ranking will. Add in the deformities we saw in the advisor, that got worse after all the resurrections. There is a loss of power each resurrection, as well as mutations.
So the demons want to get him without having to resurrect, but the moment he dies he will be in their hands.
But yeah they cant risk it, easier to keep him who they have taken the skill away from alive and locked up. They at least know that he can't be summoned like that, not worth taking the risk.